The present invention relates to an Fe-Ni-Co alloy for a shadow mask, and more particularly relates to improvement in function of a low thermal expansible, thin Fe-Ni-Co alloy plate used for production of shadow masks in the electronic and electric industries.
A shadow mask used for covering inside of a display screen of a cathode-ray tube for an electronic or electric device is provided with openings for directing electron beams to a given part of fluorescent plane. Conventionally, soft-steel plates made of, for example, low-carbon rimmed steel or low-carbon Aluminum-killed steel have been used for production of such shadow masks.
As well known, doming usually occures in radiation of electron beams through a shadow mask. That is, some of the radiated electron beams do not go through the opening of the shadow mask and causes its thermal expansion through heating by electron impingement upon the shadow mask. Being influenced by this thermal expansion, electron beams passing through the holes of the shadow mask cannot be correctly directed to a given part of the fluorescent plane. This fault in electron radiation is called "doming" in the field of the electronic and electric industries.
As stated above, soft-steel plates are widely used for production of shadow masks. Relatively high thermal expansion of these materials, however, tends to cause high degree of doming in electron radiation. Stated otherwise, these conventional materials are in general very poor in doming characteristics.
In an attempt to decrease doming in electron radiation, it is proposed to form shadow masks from Fe-Ni invar alloy. This alloy generally contains 36% by weight of Ni and Fe in balance.
In the field of electronic display devices, there is increasing demands for ultra-fine displays and flat-face configurations with high vision development. These recent trend, however, produces lots of difficult problems in production of shadow masks.
As stated above, soft-steel such as low-carbon rimmed steel and low-carbon Aluminum-killed steel are unsuited for production of shadow masks because of their high thermal expansion. They cause high degree of doming. This defect is amplified with increase in demand for ultra-fine display. Fe-Ni invar alloy plates are poorer in etching adaptability (etching-factor) than soft-steel plates and, as a consequence, it is very difficult to make mask openings at a small pore-pitch. The pore-pitch of the mask openings can be made smaller by reducing the mask thickness. A reduced mask thickness, however, lowers the rigidity of the mask after pressing and, as a consequence, the product, i.e. a Braun tube, including the mask cannot well withstand mechanical shocks applied thereto in transportation and/or use. The thermal expansion of an Fe-Ni invar alloy plate is from 1/7 to 1/10 of that of a soft-steel plate. However, the degree of doming of an Fe-Ni alloy plate is only about 1/3 of that of a soft-steel. This poor effect in reduction of doming is resulted from the relatively low thermal conductivity and large specific resistance of the Fe-Ni invar alloy.